Epidemiological studies have consistently found cleaners to be at increased risk of new-onset asthma,1 ,2 the risk seems to be particularly high in domestic cleaners using spray bleaches.2 Due to the large numbers employed in cleaning, they constitute a significant proportion of the population attributable risk of occupational asthma. It has, however, been very difficult to identify the precise cause of domestic cleaners' asthma from specific inhalation challenge (SIC) tests. Chlorine-based substances form the basis of many domestic (sodium hypochlorite) and healthcare (sodium dichloroisocyanurate) cleaning agents. Approximately one third of workers exposed to cleaning agents notified to the UK Health and Safety Executive SWORD (surveillance of work-related and occupational respiratory disease) database with occupational asthma are reported to be due to chlorine-releasing agents (personal communication from Raymond Agius, Institute of Occupational and Environmental Health, University of Manchester). At the Birmingham Occupational Lung Disease Service UK, we have seen 12 healthcare professionals and cleaners with occupational asthma who were exposed to chlorine-releasing agents. In the four investigated with SICs, testing was negative to chlorine-releasing tablets mixed with cold water, and other agents to which they were exposed. Chloramines, particularly nitrogen trichloride, have however been shown to be the cause of asthma in swimming pool attendants and teachers where the chlorine in the water reacts with nitrogen in the urine and sweat of swimmers.3 We hypothesised that similar reactions may be responsible for the asthma in domestic cleaners and healthcare workers and therefore carried out further investigations on the fourth …